PAPERS ON GEOGRAPHY AND GEOLOGY 175 



luting a river for the city next below is a great deal like 

 poisoning your neighbor's well. Cities that have deep 

 wells or spring supplies are likely to show least responsi- 

 bility. One might cite Bockford, Freeport, Peoria, 

 Aurora and Jx>liet. 



Illinois is not all equally favored. South of a line 

 running east and west through Champaign, water, ac- 

 cording to the 1914 Bulletin, is seldom obtained in 1 

 quantities either from deep rock or the drift, and such 

 ground water usually is very hard. Some exceptions 

 exist in Southern Illinois; but it is frequently the ease 

 that there is no choice but to adopt a surface supply in 

 the south half of our State. This means the impounding 

 of water, and this enhances the importance of main- 

 taining the streams in an uncontaminated condition. 



On the other hand many of the cities in the north half 

 of the State are not obliged to resort to ground water 

 due to the accessibility of deep rock supplies. These 

 waters, as a rule, while tit for domestic uses, are not 

 adapted to industrial purposes without treatment, due 

 to the mineral content. Impounded surface water is gen- 

 erally much preferred and hence the large use of im- 

 pounded water for boiler and other mechanical uses. 



In the north part of the State two formations are 

 gnized as fairly sure sources of supplies, and water 

 may be found in others but not with the same degree of 

 certainty. Sometimes it may be too heavily charged 

 with minerals even for domestic uses. These two form- 

 ations are the St. Peters and the Pottsdam. The former 

 underlies much of six states and is regarded as one of 

 the most remarkable water rocks in the world. Scores of 

 municipalities procure their supplies from this forma- 

 tion, but the State reports indicate that constant pump- 

 ing is gradually lowering the water level. The Potts- 

 dam formation lies below the St. Peter's and is sepa- 

 rated from it by the Lower Magnesium. Observation, 

 however, indicates that the water level in this is also 

 being slowly lowered. 



Thus there is raised the question whether these rock 

 supplies are inexhaustible and whether finally a dense 

 population will not even in the north part of the State 



